The invention relates to an interface circuit for connecting a first apparatus to at least one second apparatus distant thereto by way of a bi-directional bus, in particular an I2C bus which comprises a data lead for the bi-directional transmission of data and a cycle lead for the bi-directional transmission of a cycle signal.
Such an interface circuit is known from EP 0 759 593 A2. The known interface circuit serves for transmitting the data of an apparatus controlled via an I2C bus to a computer and reverse via a so-called RS232 interface. The data transfer is made by way of three control leads, wherein the one lead frees the other lead in its logic condition xe2x80x9c0xe2x80x9d and blocks it in its logic condition xe2x80x9c1xe2x80x9d. In this manner data may be transmitted from the computer to the controlled apparatus in xe2x80x9cbitsxe2x80x9d and the controlled apparatus may be cycled.
Generally with the I2C bus, which consists of two leads SDA and SCL transmitting data and cycle signals in both directions, no error recognition mechanisms with respect to hardware are present. The recognition, and where appropriate alleviation, of transmission errors may only be realized through software (cf. the publication of xe2x80x9cThe I2C busxe2x80x9d of Ludwig Brackmann in ELRAD 1991, booklet 5, pages 44-47). The mentioned publication also specifies that the I2C bus serves as the communication link between the integrated circuits of a circuit board or between several circuit boards of an apparatus. In this application the I2C offers advantages with respect to the small surface area occupied on the circuit board, low operative costs and high reliability.
With medical applications there is often the requirement to exchange data or control commands between apparatuses distanced from one another. This is, for example, the case with a TEM insufflation system (TEM-transanal endoscopic microsurgery) subdivided into two apparatuses. The known apparatuses in the application must be connected to one another and be able to mutually receive and transmit information. With these types of medical applications it is extraordinarily important that all data and control commands are transmitted in an undisturbed and genuine manner. Since the apparatuses are to be as small and as inexpensive as possible, there is therefore no opportunity for software solutions for the recognition or alleviation of transmission errors. Such transmission errors may not only occur by electric coupling in the leads from the outside but also by potential differences between the apparatuses. Security against malfunctioning implemented through software, if it could successfully be carried out at all, would have too great of an expense.
Against this setting of the problem, it is the object of the invention to so form the interaction of apparatuses which are controlled by an I2C bus and which are distanced from one another, such that in a physical way malfunctioning of the data exchange is ruled out.
According to the invention such a solution is effected in that the data signal SDA and the cycle signal SCL of the I2C bus, before reaching the transmitter/receiver component and the transmission leads to the distanced apparatus, in each case are separated into a transmitting branch and a receiving branch and that, via a differential transmitter/receiver component, the signals arc transmitted between the apparatuses via differential collection leads.
At the differential transmission leads there are thus present identical signals which are, however, opposite in their polarity so that, based on a difference formation between the signals, potential data errors are eliminated. The transmitter/receiver components are terminated with characteristic impedance of the transmission leads in a manner free of reflection, by which means longer connection leads of several meters to a few kilometers may also be used.
The separation in each case into a transmitting and receiving branch is advantageously carried out by two Opto-couplers which are decoupled from one another via fast diodes and which at the same time also achieve a galvanic separation of the apparatuses communicating with one another.
These Opto-couplers are thus connected between the apparatus to which the interface is allocated and the differential bus driver. After the Opto-coupler, the now occurring signal is inputted to the differential bus driver (i.e. the transmitter/receiver component). The receiver of the differential bus driver is switched such that it always monitors the signals on the I2C bus. The transmitter, however, is by way of logic circuitry only activated when a xe2x80x980xe2x80x99 is transmitted.
Of course, provided with the interface circuit according to the invention, not only two but more than two apparatuses may communicate with one another. Each apparatus must have the same interface (circuit arrangement). The multimaster capability and the transmission frequency of the I2C remain intact.